1. Field of the Invention
The present invention is related to a method of making an MO call using a user equipment in an IOPS dual PLMN mode of operation, and more particularly, to a method of making an MO call using a user equipment in an IOPS dual PLMN mode of operation by provide PLMN precedence.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has developed a universal mobile telecommunications system (UMTS) which adopts a wideband code division multiple access (WCDMA) technology capable of providing high frequency spectrum utilization, universal coverage, and high-speed/quality multimedia data transmission. In the UMTS, a radio access network known as a universal terrestrial radio access network (UTRAN) includes multiple Node-Bs (NBs) for communicating with multiple user equipment (UE). Furthermore, a long-term evolution (LTE) system is now being developed by the 3GPP in order to further improve performance of the UMTS to satisfy users' increasing needs. The LTE system includes a new radio interface and radio network architecture which provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved UTRAN (E-UTRAN) includes multiple evolved NBs (eNBs) for communicating with multiple UEs and a core network (CN) which includes a mobility management entity (MME), a serving gateway (SGW) and other devices for non-access stratum (NAS) control.
In 3GPP Release 13, a study item was studied on isolated E-UTRAN operations for public safety (IOPS) in support of mission critical network operation for public safety (PS). The main idea laid behind this IOPS study item is to investigate needs and requirements for E-UTRAN operations when encountering connection problems with the macro evolved packet core (EPC).
For the scenario that an eNB has no backhaul to the macro EPC, the general IOPS architectural requirements include:                (a) When a UE accesses the eNB in an IOPS mode of operation, a local IP connectivity and transport public safety service shall be provided to the UE by a local EPC connected to the eNB, if authorized.        (b) An eNB that supports IOPS may enter the IOPS mode of operation after it detects lack of S1 connectivity to the macro EPC. The decision by an eNB to enter the IOPS mode of operation shall be made in accordance with the local policies of the radio access network (RAN) operator.        
According to related 3GPP document, the macro EPC is the EPC which serves an eNB in normal mode of operation, while the local EPC is an entity which provides functionality used by an eNB in IOPS mode of operation in order to support public safety services. A nomadic eNB (NeNB) is a nomadic cell which may include a base station, antennas, microwave backhaul and support for local services. The NeNB is intended for PS use by providing coverage or additional capacity where coverage was never present (e.g. forest fire or underground rescue) or where coverage is no longer present (e.g. due to natural disaster). An isolated E-UTRAN can be created by either an E-UTRAN without normal connectivity with the macro EPC or deployed NeNBs with E-UTRAN functionality provided by a local EPC.
FIG. 1 is a diagram illustrating an IOPS architecture of an E-UTRAN 100 in response to an outage event within the network. An isolated E-UTRAN 100A can be created from the E-UTRAN 100 following an event which isolates a part of the E-UTRAN 100 from normal connectivity with the macro EPC or following deployment of standalone E-UTRAN NeNBs within a part of the E-UTRAN 100. The isolated E-UTRAN 100A may include (1) operation with no connection to the macro EPC; (2) one or multiple eNBs; (3) interconnection between eNBs; (4) limited backhaul capacity to the macro EPC; and (5) the services required to support local operations (e.g. group communication) in the case of no network coverage or of limited network coverage.
Referring to FIG. 1, the isolated E-UTRAN 100A can be created when the outage event has been occurred within the wireless communication system. The eNBs in the normal-mode E-UTRAN 100B are connected to the macro EPC by backhaul connection and the macro EPC is connected to the application server. When the isolated E-UTRAN 100A is created, the eNBs in the isolated E-UTRAN 100A are able to support services required for local operations even though a connection to the normal mode E-UTRAN 100A, as well as to the macro EPC, has been cut off.
FIG. 2 is a diagram illustrating UE behavior in an IOPS-capable wireless communication system. For illustrative purpose, it is assumed that a coverage area 20A is served by an IOPS-mode eNB1 (i.e., isolated from the macro EPC) and a coverage area 20B is served by a normal-mode eNB2 (i.e., connected to the macro EPC). PS UE0˜UE5 represent IOPS-capable mobile devices which can support a PS band. The PS UE1˜UE3 may be served by one or multiple cells in the IOPS-mode eNB1 and get connected to each other if the IOPS operation allows it. The PS UE4˜UE5 may be served by one or multiple cells in the normal-mode eNB2 and get connected to each other. The PS UE0 is under the coverage of both the IOPS-mode eNB1 and the normal-mode eNB2.
In an IOPS dual PLMN mode of operation, the PS UE0 is configured to register on both a PLMN of the IOPS-mode eNB1 and a PLMN of the normal-mode eNB2. Therefore, the PS UE0 is able to receive mobile terminated (MT) calls from any of the PS UE0-UE5. However, there is a need for a method of making mobile originated (MO) calls using a user equipment which is registered on both a normal PLMN and an IOPS PLMN.